1.Field of the Invention
This invention relates to semiconductor devices, and more particularly to Zener diodes and to metal-semiconductor field effect transistor (MESFET) integrated circuit fabrication techniques on gallium arsenide substrates.
2. Description of the Related Art
A Zener diode is a two-terminal semiconductor junction device with a very sharp voltage breakdown in the reverse-bias region. They have been used most frequently in voltage regulator circuits to provide a voltage reference. Whereas most semiconductor diode applications make use of the pn-junction rectification characteristics for the functions of rectification, switching or mixing, in the Zener diode the normal rectifying characteristic of the junction is of little interest. Rather, it is the electrical breakdown in the normally blocking polarity of the junction that is most critical.
A typical Zener diode voltage-current characteristic is shown in FIG. 1. In forward bias it exhibits the usual current response typical of other diodes. In reverse bias, however, a sharp breakdown voltage point is encountered at 2, at which a large current flow begins. With classical Zener diodes the voltage breakdown is a result of tunneling, but most diodes currently described by this name actually break down by the avalanche process. Detailed information on Zener diode operation is generally available, such as in "Handbook of Semiconductor Electronics", edited by Lloyd P. Hunter, McGraw-Hill Book Company, 1970, pages 3-37-38.
A present, no practical process has been made available for fabricating a Zener-type device in a gallium arsenide substrate in a manner which is readily compatible with existing MESFET integrated circuit technology. The availability of diodes with sharp breakdown characteristics would be useful for level shifting, typically by a few volts, in MESFET circuits such as analog-to-digital converters, and potentially for providing stable reference voltages "on chip". To date level shifting in gallium arsenide integrated circuits has been accomplished by stacking up several Schottky diodes. In this type of diode a rectifying junction is formed at the interface of a deposited metal layer and a semiconductor substrate. They exhibit rectification characteristics similar to those of pn-junction diodes, but unlike pn-junction diodes they do not exhibit any minority-carrier charge-storage capacitance. When forward biased, the Schottky diodes drop about 0.7 volts each. Although this configuration approaches the desired breakdown characteristic, it is unsatisfactory because the stack has a relatively high series resistance, and also a high temperature coefficient of voltage drop.